1. Field Of The Invention
This invention relates to a method of improving the operation of an axial-flow fan, and to an improved axial-flow fan, and to an improved axial-flow fan.
2. Description Of The Prior Art
It is known that operation of an axial-flow fan has a tendency to become unstable, when the flow is reduced from a normal operational level by reason of turbulence in the passage of medium around the rotor blades of the fan.
It is also known that an axial-flow fan generates a pure sound in the form of a siren-like noise, at a frequency proportional on the one hand to the speed of rotation of the fan and moreover to the number of its blades. The result is an acoustic nuisance which is manifest for example in power stations and in aircraft jet propulsion units which have such fans.
Attempts have been made to reduce this nuisance by adopting one or more of the following:
ADDITION OF A SILENCER UPSTREAM AND/OR DOWNSTREAM OF THE FAN;
DISPOSITION OF A HOOD AROUND THE FAN;
ADDITION OF AN INTERNALLY HONEYCOMBED ENVELOPE AT THE LEVEL OF THE ROTOR;
PROVISION OF RESONATORS UPSTREAM OF THE ROTOR;
DISPOSITION OF A GRILLE UPSTREAM OF THE FAN; THIS REQUIRES OVER-DIMENSIONING OF THE FAN.
It will be appreciated that these prior devices have disadvantages, particularly in that they involve bulky equipment and reduce fan efficiency.
French Pat. No. 1,505,043 discloses extension of the stable operating zone of a compressor or of an axial blower, and Swiss Pat. No. 437,614 discloses a reduction in noise in an axial-flow fan. These two prior Patents provide for use of an additional flow to modify turbulence in the flow of medium in the region of the fan blades, and the means used to create this additional flow comprise slots or channels in the blades, the slots or channels not being at the same time supplied with medium drawn off upstream of the fan and adapted to convey the medium to the periphery of the blades. On the contrary, if one examines FIG. 5 of the Swiss Patent, it would appear that drawing-off of additional medium through the pipe 15 occurs not only upstream but downstream of the rotor.